1. Field of the Invention
This invention relates to a process for the electrochemical treatment of plant biomass material to convert it into its constituent parts. More particularly, this invention relates to a process for converting lignocellulosic material into cellulose, hemicellulose and lignin.
2. Discussion of the Prior Art
Biomass, particularly agricultural byproducts and other plentiful sources of otherwise wasted lignocellulosic material, is being studied as a source of fuels, chemicals and polymers to supplement petroleum-derived materials. Because lignocellulose is a complex structure containing cellulose and hemicellulose bound with lignin, it must usually be treated and degraded in order to obtain more useful items.
Such useful items obtained from the processing of lignocellulose include organic acids such as acetic acid, lactic acid, butyric acid, malic acid, succinic acid, fumaric acid, citric acid and itaconic acid, and bipolymers including xanthan gum, pullulan, poly(hydroxybutyrates) and poly(alkonates). Other chemicals produced are solvents comprising ethanol, acetone, n-butanol, isopropanol and acetaldehyde, polyhydric alcohols such as propylene glycol, glycerol and butylene glycol in addition to specialty chemicals such as amino acids, enzymes and antibiotics.
Cellulose is a long-chain carbohydrate polymer composed of anhydrogluclose units and has the empirical formula: EQU C.sub.6 H.sub.10 O.sub.5
It is the major constituent of cell walls in higher plants, and is also found in lower plants such as mosses, ferns, algae and fungi. A dimer of cellulose, cellobiose, is formed during the conversion to sugars and will inhibit the degradation process.
Hemicellulose is a short-branched carbohydrate polymer occurring in the cell walls of plants in conjunction with cellulose and lignin. It is ordinarily extracted from plant materials with aqueous solutions of sodium hydroxide.
Lignin is a random-structured polymer formed from substituted phenylpropane units and is present in most plant materials. The proportion of lignin varies in different plant sources and in different parts of the same plant. One of its functions is to glue or cement the plant together. Although readily oxidized, it essentially cannot be hydrolyzed by acids. Lignin will also block the activity of cellulase, an enzyme which attacks the cellulose substrate. Lignin must usually be degraded or separated from the cellulosic portion of the biomass prior to significant hydrolysis of the cellulose.
While it may be conceptually simpler to hydrolyze the biomass directly, it is actually more efficient to first at least partially degrade or remove the lignin from the biomass before hydrolyzing the remaining cellulose or further processing the lignin. Hydrolysis of the cellulose substrates is the primary rate-limiting step in the conversion of biomass to useful fuels and chemicals. The rate of hydrolysis is in turn dependent upon the rate at which the biomass becomes exposed to the actions of acids or enzymes.
Several pretreatments are known to enhance the activity of these actions. The most common form of pretreatment is a physical destruction such as chipping, pulverizing, cutting or milling. This serves to alter the cellulose structure as well as expose more surface area of the lignocellulose. The benefits of a physical grinding as a pretreatment method are well known, and are shown for example in U.S. Pat. No. 2,801,955, a process for the extraction of hemicellulose, and U.S. Pat. No. 3,523,911, an acid-steam separation of lignocellulose. Pretreated material is more susceptible to enzyme action, acid hydrolysis or separation of the component parts by pulping, steam treatment, alkaline extraction, and the like.
After pulverization, the substrate typically undergoes a further separation or processing step. This can include acid or enzyme hydrolysis of the cellulose to produce simple sugars which can be fermented and distilled to alcohols. The lignin can be converted to dispersants or used as fillers, binders and resin extenders. Further processing of lignin can provide chemicals such as vanillin, methyl mercaptan and dimethyl sulfide. Hemicelluloses are often saccharified to produce sugars such as pentoses, and can have other uses in coating compositions, alcohols, xylose and xylitol, and the like. Such processing steps and techniques are more fully described in Hydrolysis of Cellulose: Mechanisms of Enzymatic and Acid Hydrolysis edited by R. D. Brown, Jr. and L. Jurasek, American Chemical Society, Washington, D.C., 1979.